Abstract

Long-range acoustic communication represents one of the main modalities to transmitinformation and maintain social relationships among distant individuals. By examiningvariation in long-range signals, we can gain insights into their function as well as theprocesses underlying the variation. Since communication and social behaviour are tightlylinked, variation in acoustic signals can reflect behavioural patterns that are important tobetter understand the sociobiology of a species. Moreover, changes in vocal behaviour canalso be induced by factors interfering with acoustic communication, such as anthropogenicnoise. In my thesis, I investigated patterns of variability in the long-range male vocal displays(‘songs’) of North Atlantic blue whales (Balaenoptera musculus) at the level of thepopulation, the individual, as well as in relation to the social and behavioural context. I alsoexamined whether seismic noise affected the production of blue whale ‘calls’, transientdiscrete vocalisations used by both sexes in short-range social contexts. Blue whales areinvaluable for the study of variation in long-range signals because they live in a highly fluidsocial environment, and their songs - redundant, patterned sequences of infrasonic sounds -can be heard over hundreds of kilometres. I collected simultaneous behavioural observationsand acoustic recordings in the field and analysed the temporal and acoustic structure of bluewhale songs. I found that multiple features differed between the songs of blue whales fromthe eastern and western North Atlantic. It is thought that due to the extensive singing duringtheir basin-wide roaming, blue whales throughout the North Atlantic may be physicallyand/or acoustically sympatric. The song divergence found here was likely dependent onecological factors, but could also have been driven by mate choice. In fact, songs are thoughtto be male reproductive displays, suggesting that female preferences for specific traits couldhave accounted for the observed differences. Blue whale songs also showed inter-individualvariation and may thus convey information relevant to distinguishing between singers.Considering that multiple senders can be active at the same time, individual song differencesare likely used by conspecifics to assess individuals. Since blue whales roam highlydispersed, singing might play a role in mediating inter-individual interactions from a distance(e.g. mate attraction). This hypothesis was supported by the fact that songs were alwaysproduced by single steadily moving males and appeared to be directed to an unknownaudience. In contrast, I found that in stationary situations (feeding, socialisation) and in thepresence of females nearby, infrasonic sounds composing a song were emitted singly or inshort irregular sequences (fragments). This suggests that in a stationary short-range situation,the repetition and redundancy found in songs is superfluous because potential receivers are inclose physical and visual range. Fragments might suffice to indicate the presence of asignaller and efficiently transfer individual information.Finally, I found that blue whales consistently increased call production when exposed toseismic survey activity. The observed response presumably represents a compensatorybehaviour to the rise in ambient noise from the seismic survey. Increasing call productioncould enhance the probability of receivers to detect the signal (‘information theory’). Sincethe response was observed at low noise levels, this finding has substantial management andconservation implications.This thesis increases our knowledge about the use of long-range vocalizations in bluewhales, particularly with respect to mate attraction and also about compensatory vocalbehaviours in response to interfering anthropogenic noise. It shows that variation can occur atmultiple levels of the signal and that blue whale songs are multicomponant signals that mighthave evolved to cope with a variable socio-ecologial environment.

Long-range acoustic communication represents one of the main modalities to transmitinformation and maintain social relationships among distant individuals. By examiningvariation in long-range signals, we can gain insights into their function as well as theprocesses underlying the variation. Since communication and social behaviour are tightlylinked, variation in acoustic signals can reflect behavioural patterns that are important tobetter understand the sociobiology of a species. Moreover, changes in vocal behaviour canalso be induced by factors interfering with acoustic communication, such as anthropogenicnoise. In my thesis, I investigated patterns of variability in the long-range male vocal displays(‘songs’) of North Atlantic blue whales (Balaenoptera musculus) at the level of thepopulation, the individual, as well as in relation to the social and behavioural context. I alsoexamined whether seismic noise affected the production of blue whale ‘calls’, transientdiscrete vocalisations used by both sexes in short-range social contexts. Blue whales areinvaluable for the study of variation in long-range signals because they live in a highly fluidsocial environment, and their songs - redundant, patterned sequences of infrasonic sounds -can be heard over hundreds of kilometres. I collected simultaneous behavioural observationsand acoustic recordings in the field and analysed the temporal and acoustic structure of bluewhale songs. I found that multiple features differed between the songs of blue whales fromthe eastern and western North Atlantic. It is thought that due to the extensive singing duringtheir basin-wide roaming, blue whales throughout the North Atlantic may be physicallyand/or acoustically sympatric. The song divergence found here was likely dependent onecological factors, but could also have been driven by mate choice. In fact, songs are thoughtto be male reproductive displays, suggesting that female preferences for specific traits couldhave accounted for the observed differences. Blue whale songs also showed inter-individualvariation and may thus convey information relevant to distinguishing between singers.Considering that multiple senders can be active at the same time, individual song differencesare likely used by conspecifics to assess individuals. Since blue whales roam highlydispersed, singing might play a role in mediating inter-individual interactions from a distance(e.g. mate attraction). This hypothesis was supported by the fact that songs were alwaysproduced by single steadily moving males and appeared to be directed to an unknownaudience. In contrast, I found that in stationary situations (feeding, socialisation) and in thepresence of females nearby, infrasonic sounds composing a song were emitted singly or inshort irregular sequences (fragments). This suggests that in a stationary short-range situation,the repetition and redundancy found in songs is superfluous because potential receivers are inclose physical and visual range. Fragments might suffice to indicate the presence of asignaller and efficiently transfer individual information.Finally, I found that blue whales consistently increased call production when exposed toseismic survey activity. The observed response presumably represents a compensatorybehaviour to the rise in ambient noise from the seismic survey. Increasing call productioncould enhance the probability of receivers to detect the signal (‘information theory’). Sincethe response was observed at low noise levels, this finding has substantial management andconservation implications.This thesis increases our knowledge about the use of long-range vocalizations in bluewhales, particularly with respect to mate attraction and also about compensatory vocalbehaviours in response to interfering anthropogenic noise. It shows that variation can occur atmultiple levels of the signal and that blue whale songs are multicomponant signals that mighthave evolved to cope with a variable socio-ecologial environment.

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